1. Field for the Invention
This invention relates to an antenna input circuit of an electronic tuning type AM receiver for automobiles.
2. Description of the Prior Art
An antenna input circuit for a medium wave AM receiver which is mounted in an automobile, is usually the electronic tuning type using a variable capacitance diode. As, the antenna of an AM receiver of this kind, a whip antenna is widely employed. Since the whip antenna is capacitive, it is necessary that the variable capacitance diode in the electronic tuning type AM receiver have a sufficiently large capacitance variation range with respect to the antenna capacitance. But the capacitance variation range of the variable capacitance diode is not large enough; therefore, there have been various proposals for tuning in the medium-wave frequency range from 525 to 1605 KHz, and further, a problem of low S/N ratio has been presented.
In the past, use has been made of (a) a non-tuning input type, which amplifies the antenna output by a field effect transistor or the like and then applies the amplified signal to a tuning circuit, or (b) a tuning input type, which employs two variable capacitance diodes, one being connected in series with a tuning transformer and the other connected therewith in parallel. However, since the non-tuning input type (a) amplifies all signals received by the antenna, it has the defect of cross modulation, whereas the tuning input type (b) is free from cross modulation but defective in that the use of two variable capacitance diodes leads to an increased manufacturing cost and difficult tracking adjustment.